1. Field of the Invention
The present invention relates to homogeneous glass materials formed from rare earth oxides and aluminum oxide. In particular, the present invention relates to rare earth oxide-aluminum oxide glass compositions in which the addition of lanthanum oxide eliminates the phase separation observed in prior art glasses, allowing homogeneous glasses to be prepared. The invention also relates to homogeneous glasses formed by rare earth oxides and aluminum oxide that contain smaller amounts of the glass forming agents, SiO2 and/or B2O3 than are used in prior-art glasses. Such glasses comprise mixtures of rare earth element oxides (RE2O3), i.e., oxides of the elements with atomic numbers of 21, 39, 57-71: Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, combined with aluminum oxide, Al2O3, and, optionally, additives comprising oxides of other elements.
2. Description of the Prior Art
Glasses have numerous applications in sensors, electrical insulators, instruments, optical devices, optical waveguides, and components that require uniform optical properties, uniform density, and freedom from asperities and defects that would cause light scattering. Glasses containing selected rare-earth ions or xe2x80x9cdopantsxe2x80x9d, for example, Er, Nd, Yb, Pr, Ho, Eu, are of particular value due to their applications in lasers, amplifiers, optical switches, optical filters, and optical components for light transmission at ultraviolet, visible, and infra-red wavelengths. Compared to crystals, glass xe2x80x9chostsxe2x80x9d can enable much broader bandwidths and greater pulsed power output for laser transitions by providing a range of electronic environments for the laser active ions, thereby extending the range of wavelengths over which laser sources and amplifiers can be operated.
Much effort has been invested to develop glasses for commercial applications. The presence of random fluctuations (often called xe2x80x9cstriaexe2x80x9d) in composition, density, index of refraction and other properties makes a glass practically useless for optical applications, even window glass, due to light scattering and the unpredictable optical path through the material. Striae, and other inhomogeneities, also limit the utility of a glass in applications where uniform mechanical and thermal properties are required. As a result, development of homogeneous, single-phase glasses has been a priority. In addition, development of glasses with high refractive index, glass waveguides and fibers, infrared transmitting glasses, and filter glasses has been driven by continually increasing application requirements.
The prior art in oxide glass making can be described in terms of the xe2x80x9cfamiliesxe2x80x9d of glass based on certain glass forming agents that are currently used. The prior art xe2x80x9cfamiliesxe2x80x9d of glass are based on compositions which contain substantial amounts of one or more of the glass forming agents: silica, SiO2; arsenic oxide, As2O3; boria, B2O3; germania, GeO2; phosphorus oxide, P2O5; telluria, TeO2; and vanadia, V2O5. The widely used silica-based crown and flint glass families were developed in the early 1900s by Schott and others. Germanate glasses (based on germanium oxide, GeO2) were developed in the 1920s, phosphate glasses (based on phosphorus oxide, P2O5) were developed in the late 1930s. Heavy metal borate glasses with high refractive index were developed by Morey in the 1940s. Tellurite glasses (based on tellurium oxide, TeO2) were developed in the 1950s.
Prior art relating to rare earth aluminum oxide glasses includes Sedykh, et al. [Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy, Vol. 11, No. 6, pp. 1153-1154, 1975], who report glass forming compositions in the Y2O3xe2x80x94Al2O3xe2x80x94SiO2 system. The glass forming composition with the least amount of SiO2 that is reported by Sedykh, et al. contains 40.3 molar % SiO2, 43.4 molar % Al2O3 and 16.3 molar % Y2O3. The glasses reported by Sedykh, et al. contain from 16 to 33 molar % Y2O3. Day and Ehrhardt, in U.S. Pat. No. 4,789,501, disclose glass microspheres containing aluminum oxide, yttrium oxide and silicon oxide. The compositions of the glass microspheres of Day and Ehrhardt, and of the bulk glasses of Sedykh, et al. are within a region of the ternary Y2O3xe2x80x94Al2O3xe2x80x94SiO2 composition diagram that is given in weight percent and converted to molar percent in Table 1.
McPherson and Murray, in U.S. Pat. No. 5,747,397, also disclose glass containing rare earth oxides and aluminum oxide with additions of glass-forming agents and other oxides. The glasses disclosed by McPherson and Murray contain 30-70 molar % of oxide glass forming agents (SiO2, GeO2, B2O3, and P2O5), 0-25 molar % of Al2O3, 20-42 molar % of RE2O3 (Y2O3, Er2O3, Nd2O3, La2O3, Gd2O3, and Yb2O3), and 0-28 molar % of other oxides (TiO2, K2O, ZrO2, ZnO, Nb2O3, and Ga2O3). The glasses claimed by McPherson and Murray contained from 48-66 molar % of TiO2+SiO2+GeO2+B2O3, i.e., no more than 52 molar % of Al2O3+RE2O3. The compositions disclosed by McPherson and Murray contained no more than 55 molar % of Al2O3+RE2O3.
Eberlin, in U.S. Pat. 2,206,081 discloses optical glasses comprised of heavy metal oxides and B2O3 as the glass forming agent. The glasses disclosed by Eberlin contain 37-70 molar % of B2O3, no Al2O3, 10-32 molar % of La2O3, and 20-39 molar % of other oxides (ThO2, Ta2O5, ZrO2, Na2O, and LiNO3) Glass materials with compositions in the range 24 molar % to 32 molar % Y2O3 with the balance Al2O3 have been reported in the scientific literature. These yttrium oxide-aluminum oxide glasses occur as a heterogeneous mixture of two different glasses that form when the liquid is rapidly cooled. A glass of 37.5 molar % Y2O3 in Al2O3 has also been reported, but we show here that this glass is also a mixture of two different glasses.
All of the rare earth elements and aluminum exhibit an oxidation state of +3. Four-, six-, and eight-coordination ionic radii for these ions are given in FIG. 1. The horizontal lines in the figure are the cation radii for 4-, 6- and 8-coordination with Oxe2x88x922 ions at which the coordinated Oxe2x88x922 anions just touch, i.e., the xe2x80x9ccriticalxe2x80x9d radii for 4-, 6-, and 8-coordination with Oxe2x88x922. According to Pauling""s rules, the local coordination most likely to occur is that of the largest polyhedron for which the cation radius exceeds the critical radius.
The prior art also teaches that optical waveguides may be made from crystalline oxide materials, such as yttrium aluminum garnet (YAG) of composition Y3Al5O12, with compositions similar to those of this invention.
Prior art glasses containing substantial amounts of rare earth oxides and aluminum oxide are either two phase glasses that are not suitable for optical applications, or they contain at least 30 molar % of the glass forming agents, SiO2 and/or B2O3.
Thus there exists a need for glasses that may readily be configured into optical waveguide forms and dimensions to be used in applications concerning guiding light waves for the purpose of operating optical devices such as lasers and optical amplifiers.
It is, therefore, an object of the invention to provide a glass that can be used in optical applications where uniform mechanical and thermal properties are required.
Another object of the invention is to provide homogeneous, single phase glasses.
Yet another object of the invention is to provide glasses that have a high refractive index.